The present invention relates to electrically driven power tools. In particular, the invention relates to a motor control circuit and method for detecting and responding to the onset of stall conditions (e.g., kickback or breakthrough conditions) by pulsing the motor to overcome the stall condition, if possible.
Electrical power tools typically employ a motor that imparts torque to the tool through a spindle. In the case of an electric drill, the motor spindle is coupled through a series of reducing gears to the chuck, which in turn holds the drill bit or other cutting or abrading tool, such as a hole saw, a grinding wheel, or the like. Power screwdrivers work on a similar principle, with the chuck holding a screwdriver bit. In both cases, the function of the reducing gears or gear train is to reduce the rotational speed of the tool while increasing the rotational torque.
Power routers are somewhat different. The cutting tool (router bit) of the typical hand-held router is typically direct-coupled to the spindle of the motor. In this case, the full rotational speed or RPM of the motor is used without gear reduction, to rotate the router bit at high speed. Reciprocating saws and jigsaws use yet another type of gear train that translates the rotational movement of the motor spindle to reciprocating movement.
Generally speaking, all of these power tools may suddenly encounter impending stall conditions at which time the output torque rapidly rises because of local changes in workpiece hardness, workpiece binding or jamming, tool obstruction from burrs and so forth. If the cause of the condition is not overcome, the tool may jam and the motor will stall. When drilling a hole with a power drill, for example, some workpieces will develop burrs on the tool exit side of the workpiece, and these burrs can engage the flutes of the drill bit, causing a rapid increase in torque as the drill bit tries to break free. In some instances, especially with metal workpieces, the burrs may actually stop drill bit rotation, causing a strong reaction torque that is imparted to the tool operator as the motor turns the tool in the operator""s hand (instead of turning the drill bit).
A related phenomenon occurs with power saws. Referred to as kickback, the cutting movement of the saw blade may become partially or fully arrested by the workpiece, often when the saw cut is approaching completion and the unsupported workpiece becomes jammed against the saw blade. With the movement of the saw blade impeded, a large motor torque is generated and, in some case, the motor may actually stall.
These conditions are hereinafter generically referred to as xe2x80x9ckickbackxe2x80x9d or xe2x80x9cstallxe2x80x9d conditions, regardless of the particular power tool involved or the specific circumstance which gives rise to the impending kickback or stall condition.
In the past, the Applicant""s assignee developed anti-kickback power tool control techniques that sensed an impending kickback condition and inhibited the coupling of power to the tool, and/or optionally applied a brake to the tool, in response to the impending kickback condition. These systems are described more fully in U.S. Pat. No. 4,267,914 to Saar, entitled xe2x80x9cAnti-Kickback Power Tool Control,xe2x80x9d issued May 19, 1981; and in U.S. Pat. No. 4,249,117 to Leukhardt et al., entitled xe2x80x9cAnti-Kickback Power Tool Control,xe2x80x9d issued Feb. 3, 1981. The anti-kickback control systems described in these patents are designed to interrupt power to the motor once the impending kickback condition occurs. In order for power to once again be coupled to the tool, the trigger switch must be fully released and then again retracted, or some other signal provided by the operator.
Although the systems described in these patents are effective in detecting and preventing a kickback condition, the response of the control circuits disclosed therein to the impending kickback condition may hamper the operator""s ability to complete the desired task. For example, if power has been interrupted due to a drill bit binding on a burr formed during breakthrough, it may be difficult for the operator to break the burr free to complete the hole without repeating the stall condition and causing re-interruption of power to the motor. Consequently, an improved control technique is needed that is not only effective in detecting and preventing a kickback condition, but is also effective in enabling the operator to possibly overcome the impediment and completing the intended task.
The present invention represents an improvement to Applicant""s prior anti-kickback technology. Instead of responding to an impending kickback condition by simply interrupting power to the tool (and/or applying a brake), the present invention effects a motor pulsing operation that, in many cases, can actually resolve or clear the breakthrough or kickback condition, so that the tool does not need to be shut down and restarted. When the invention is used with power drills, for example, the operator can keep the trigger switch actuated while the present controller senses an impending stall condition (e.g., kickback or breakthrough condition) and responds to that condition by pulsing the motor for a predetermined time period to deliver a series of torque pulses. These torque pulses each have a peak torque that is substantially greater than the average torque delivered during the series of pulses. The impact of these torque pulses may allow the tool to break through the burrs or workpiece restrictions that are causing the impending stall or kickback condition. In one embodiment, the pulses are delivered in a sequence designed to be harmonically related to the natural frequency of the gear train of the power tool. This provides even greater peak torque output by causing the gear train to oscillate between an energized state and a relaxed state at the gear train""s natural frequency.
The present invention can be used with virtually any power-driven tool. When incorporated into the motor control of a rotary saw, the motor pulsing effect may cause the workpiece to become unjammed or break through the binding condition that originally caused the impending stall or kickback.
Accordingly, the present invention provides a method and apparatus for controlling virtually any power tool having a motor that imparts torque to an output spindle when actuated by an operator actuable switch. The method involves sensing a motor parameter indicative of the onset of a stall condition. The sensed parameter can be motor current, for example, and the onset of stall can be inferred by monitoring the rate of change in motor current with respect to time. While the trigger switch remains actuated, the motor is pulsed for a predetermined period to deliver a series of torque pulses each having a peak torque substantially greater than the average torque delivered during the series of torque pulses. In many cases, these torque pulses will clear the condition that caused the impending stall. If so, then normal motor operation is resumed. If not, then the power to the motor is removed after a predetermined time to avoid burnout of the motor.
For a more complete understanding of the invention, its objects and advantages, reference is made to the following specification and to the accompanying drawings.